Semiconductor fabrication techniques often utilize a mask or reticle. Radiation is provided through or reflected off the mask or reticle to form an image on a semiconductor wafer. The wafer is positioned to receive the radiation transmitted through or reflected off the mask or reticle. The image on the wafer corresponds to the pattern on the mask or reticle. The radiation can be light, such as ultraviolet light, vacuum ultraviolet (VUV) light, extreme ultraviolet light (EUV) and deep ultraviolet light. The radiation can also be x-ray radiation, e-beam radiation, etc.
One advanced form of lithography is extreme ultraviolet (EUV) light lithography. A conventional EUV system (e.g., an optical reduction camera or stepper) utilizes an EUV radiation source, an EUV lens assembly (e.g., a condenser lens), an EUV reticle, and another EUV lens assembly (e.g., an objective lens). EUV radiation can be created at the radiation source and projected onto the reticle. The EUV reticle is typically a resonant-reflective medium including an IC pattern of absorbing material. The resonant reflective medium reflects a substantial portion of the EUV radiation in accordance with the IC pattern to the second EUV lens assembly. The lens assemblies can be an all resonant-reflective imaging system including aspheric optics at 4:1 magnification factor (e.g., a series of high precision mirrors). EUV radiation reflected off the EUV reticle is provided from the second EUV lens assembly to a photoresist coated wafer.
EUV lithography utilizes radiation in a wavelength of 5 to 70 nanometers (e.g., 11-14 nanometers). A conventional EUV lithographic system or EUV stepper provides the EUV reticle as a multilayer coated reflective mask or reticle which has an absorber pattern across its surface. The multilayer coated reflective reticle (i.e., the resonant reflective medium) can utilize molybdenum/silicon (Mo—Si) layers or molybdenum/beryllium layers (Mo—Be).
EUV lithography can employ a reflective mask consisting of a patterned absorber on a multilayer coated substrate (mask blank) that reflects a narrow band of EUV wavelengths near 13.4 nm. Such masks have the advantage of being thick and dimensionally stable; however, the use of such masks presents some challenges.
Current EUV reflective masks use silicon oxide (SiO2) as buffer layers to protect delicate multilayer reflectors. Metal layers are used as absorbers. While the silicon oxide buffer layers and metal layer absorbers are effective, the high temperature processing required by these layers can degrade the reflector (e.g., the Mo—Si or Mo—Be layers).
Conventional EUV multilayer reflectors can also be very delicate due to the need for precise interface properties to achieve high reflectance. Depositing metal absorbers and glass (SiO2) buffers add unwanted thermal cycles that can blur the interfaces and reduce reflectance.
Thus, there is a need for a low temperature layer in an EUV reflective mask. Further, there is a need to use amorphous carbon layers as absorbers or buffers or both. Even further, there is a need to make an EUV mask with high reflectance.